The principle of the method of measurement consists in connecting the two ends P and Q, of a sliding rheostat to the terminals of a dry cell, D, and balancing the potential-difference to be measured against the potential-difference between one end, P, and the slider, S, by means of a special form of enclosed capillary electrometer, E. The value of this potential-difference is read directly on a delicate voltmeter, V. The connexions, which are found ready-made in the box, have been drawn out, whereas, those to be made by the operator are shown by dotted lines. The steps to be taken by the latter, consist first in taking off the capillary electrometer and manipulating it in such a manner, that on returning it into position the capillary may be partly filled with a thread of mercury and partly with the acid. The terminals, X and Y, marked battery + and −, are connected to a dry cell, and the terminals, Z and U, marked cathode and auxiliary respectively, to the hydrogen and calomel electrode. Very careful insulation of the connexion between the terminal marked auxiliary and the calomel electrode is necessary. The hydrogen is passed through the hydrogen electrode until a constant P.D. between it and the calomel electrode is obtained. This P.D. is measured by moving the slider up and down until no movement of the mercury in the capillary electrometer is observed on depressing the key K marked electrometer. Fig. 10 is a view of the apparatus as set up for the titration of a puer liquor. H is a cylinder of compressed hydrogen;[68] I, the hydrogen electrode, dipping into the beaker C containing the liquor for titration; _b_, the burette, containing N/10 acid or alkali; II, the auxiliary electrode, the capillary of which is also seen dipping into the beaker C; P, the potentiometer box containing the sliding rheostat S and electrometer E; D, a dry battery. The acid or alkali is run in from the burette until the voltmeter shows 0·69 volts, indicating that the liquid is neutral or has a hydrogen ion concentration of 10^{-7}. [68] It is necessary to use pure hydrogen. This may now be obtained commercially from the Knowles’ Oxygen Co., Wolverhampton. [Illustration: Fig. 10.--Electrometric Apparatus.] The following table shows some results obtained on puer liquors before and after skins have been put through. _p__{H+} has been called by Sorensen the exponent of the hydrogen ion concentration C, and is defined by the equation _p__{H+} = log(1/C) i.e. it is equivalent to the logarithm of the reciprocal of the factor of normality of the solution with respect to the hydrogen ions.[69] _p__{H+} for pure water or a neutral solution is 7, corresponding to 0·69 volts. The measurements were made on filtered puer liquors, using a N/1 KCl calomel electrode as auxiliary, the capillary of the electrode being filled with 3·5N potassium chloride:-- [69] The formula for calculating _p__{H+} for the N/1 auxiliary electrode used is as follows:--